UNITY OF FLAN 7 



The venous system, too, is built upon the same general plan 

 throughout the Sanguinea. " In all sanguineous animals, the 

 nature and origin of the principal veins are the same, but the 

 multitude of smaller veins is not alike in all, for neither are 

 the parts of the same nature, nor do all possess the same 

 parts" (Cresswell, loc. at., p. 56). It .will be noticed in the 

 first and last of these three quotations that Aristotle 

 recognises the fact of correlation between systems of organs 

 between limbs and bones, and between blood-vessels and 

 the parts to which they go. 



Sanguineous animals all possess certain organs heart, 

 liver, spleen, kidneys, and so on. Other organs occur in most 

 of the classes the oesophagus and the lungs. " The posi- 

 tion which these parts occupy is the same in all animals 

 [sc. Sanguinea] " (Cresswell, loc. ct't., p. 39). 



Unity of plan is observable not only in the Sanguinea, 

 but also within each of the other large groups. Aristotle 

 recognises that all his cuttlefish are alike in structure. 

 Among his Malacostraca he compares point by point the 

 external parts of the carabus (Palinurus], and the astacus 

 (Honiarus}, and he compares also the general internal 

 anatomy of the various " genera " he distinguishes. As 

 regards Testacea, he writes, " The nature of their internal 

 structure is similar in all, especially in the turbinated animals, 

 for they differ in size and in the relations of excess ; the 

 univalves and bivalves do not exhibit many differences" 

 (Cresswell, loc. tit., p. 83). There is an interesting remark 

 about " the creature called carcinium " (hermit-crab), that it 

 " resembles both the Malacostraca and the Testacea, for this 

 in its nature is similar to the animals that are like carabi, 

 and it is born naked" (Cresswell, loc. '/., p. 85). In the 

 last phrase we may perhaps read the first recognition of the 

 embryological criterion. 



With the recognition of unity of plan within each 

 group necessarily goes the recognition of what later mor- 

 phology calls the homology of parts. The parts of a horse 

 can be compared one by one with the parts of another 

 viviparous quadruped ; in all the animals belonging to the 

 same class the parts are the same, only they differ in excess 

 or defect these remarks are placed in the forefront of the 



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